Commercial devices which effectively handle gas containing media or suspensions, such as paper pulp, at medium consistency, that is at about 6-15% solids consistency, are known. It is also known that air or, more generally gas, if present in the fiber suspensions causes problems in almost all process stages in the pulp and paper industry. When pulp is pumped, mixed, screened, washed or otherwise handled without excess gas significant savings in equipment, power consumption and the like can be achieved. For instance, one device which has been particularly successful in allowing handling of gas-containing medium consistency fiber suspensions is a fluidizing centrifugal pump which simultaneously pumps and degasses the suspension. Typically, such pumps utilize a separate vacuum pump, piping from the centrifugal pump to the vacuum pump, a separate motor and motor mount for the vacuum pump, etc., in order to exhaust the gas which has been separated from the suspension so that the suspension may be effectively pumped by the pump impeller.
U.S. Pat. No. 3,230,890 discloses a centrifugal pump for removing gas from low consistency suspensions or from water having either a built-in vacuum pump or an external vacuum pump.
A fluidizing centrifugal pump having a built-in vacuum pump is disclosed in U.S. Pat. No. 4,776,758. FIG. 1 illustrates the prior art centrifugal pump, with the volute being omitted, provided with a vacuum pump on the same shaft as impeller. The characteristic features of the prior art pumps on the market today and which have not, however, proven to be successful due to some shortcoming in the structure thereof, are disclosed in detail in the following. The prior art pump of FIG. 1 has a fluidizing impeller 12 rotating in an ordinary medium consistency pump housing. The impeller 12 has through bores 14 for allowing the air accumulated at the front side of the impeller 12 to be drawn by means of the vacuum pump 10 to the back side of the impeller 12. The impeller has also so-called back vanes 16 on the back side thereof for separating the fiber suspension from the medium being drawn through the openings 14 in the impeller plate 18. The main purpose of the back vanes 16 is to pump the fiber suspension back to the pump volute and thus prevent the fibers from entering the vacuum pump 10, as the risk of damaging the vacuum pump 10 rises dramatically if the fibers are allowed to enter the vacuum pump 10. The vacuum pump 10 is a so-called liquid ring pump which has been arranged on the pump shaft 20 behind an intermediate plate 22 in which only a narrow ring-shaped duct 24 is provided which duct surrounds the shaft 20 or the impeller extension 26 for allowing the gas to flow towards the vacuum pump. The intermediate plate 22 is also provided with a ring-shaped channel 28 and a narrow duct 30 leading thereto for introducing make-up air to the vacuum pump while the pump is running. The duct 30 is connected via channel 32 to a vacuum regulating valve (not shown). The vacuum pump housing 34 is provided with a conduit 36 for feeding liquid to the liquid ring pump 10 for maintaining the amount of liquid substantially constant therein. Conduit 36 is connected to the outer, eccentric circumference 38 of the liquid ring pump 10. In other words, the conduit 36 leads exclusively and directly to the liquid ring. The suction opening for the liquid ring pump 10 is provided, naturally, on the side of the centrifugal impeller 12. The discharge channel (not shown) for the gas to be removed from the pump 10 is arranged at the opposite side of the vacuum pump 10, i.e. on the back side of the vacuum pump relative to the centrifugal impeller 12.
Various problems have, however, been encountered with the pump in operation today. For example, the air removal capacity has been significantly lower than required, i.e. the vacuum created has not reached a sufficiently high level. Also, the discharge pressure of the vacuum pump has been found to be too low. In some cases, it is desired to introduce the material discharged from the vacuum pump, a mixture containing mainly gas but also some fibers, into the top portion of a mass tower to recover the fibers. If, however, the discharge pressure of the vacuum pump is too low the pumped material cannot be conveyed to the top of the mass tower, and an additional pump must be installed for that purpose. Most importantly, the open annular volume in the intermediate plate 22 of prior art pump has a tendency to become clogged by the fibers.
In the prior art pump the axial gap 40 between the vanes 42 of the vacuum pump 10 and the axially adjacent walls 44 of the vacuum pump housing are not adjustable but are positioned with a distance or clearance of about 0.4 mm. The reasons for such relatively large clearance is the fact that there are a number of factors which render it is impossible to further decrease the clearance 40 as the various components of the pump are installed on the shaft or around the shaft starting from the drive end 46 of the shaft. Thus, the dimensions of the components effect the clearance 40. The result of too wide a clearance is, of course, an insufficient vacuum. Another reason for the wide clearance 40 may also be the fact that the shaft 20 of the pump tends to flex somewhat during operation creating the risk of mechanical contact between the vacuum pump vanes and the housing walls 44. Thus, the large clearance 40 has been provided intentionally to ensure long lasting operation of the pump.
The pump in accordance with the present invention is designed to eliminate most or all of the above problems. Accordingly the pump of the present invention is provided with an intermediate plate separating the centrifugal pump from the vacuum pump which is, preferably, a liquid-ring pump. A non-annular volume is provided within this intermediate plate which non-annular volume communicates with the inside of the vacuum pump chamber and the outside of the pump for permitting gas as well as liquid to flow through the non-annular volume into the vacuum pump chamber. The non-annular opening provided in the intermediate plate is located at or in close proximity to the shaft in accordance with the pressure distribution present in the volute. For example, if located at the point of highest pressure, less vacuum or no additional vacuum is required to discharge the gas from the volute. Alternatively, the non-annular opening may be located at the point of lowest pressure just behind the pump outlet when viewed in the direction of rotation of the pump impeller thereby preventing the fibers from being drawn into the volute together with the gas.
In addition, the non-annular opening is preferably connected to a non-annular volume in the intermediate plate.
In a further embodiment of a pump and in accordance with the present invention an open annular volume is arranged within the back vanes of the centrifugal impeller by providing an annular surface substantially parallel to the shaft or the impeller extension sleeve either at the back plate of the impeller or at the radially inner edges of the back vanes so that a circular space or gas flow passage is formed between one side of said annular surface and the intermediate wall or between one side of the surface and the impeller back plate depending on where the annular surface is attached. The open circular gas flow passage is useful for equalizing the pressure differences in the space between the back vanes of the centrifugal impeller.
The pump of the present invention may also be provided with means for introducing a liquid into the pump, and especially into the non-annular volume and air flow ducts of the pump for flushing these critical locations with a liquid such as flushing water and freeing the pump from fibers which otherwise tend to block the flow path of the pump. The flushing ducts may also be used to supply working liquid to the liquid ring of the vacuum pump.
The pump of the present invention also provides means for adjusting the relative axial position of the vacuum pump rotor relative to the front and rear wall of the vacuum pump chamber thereby providing significantly smaller operational clearances therebetween. This may be achieved by either adjusting the axial position of the rotor with respect to the shaft, for example, by the addition of shims between respective shoulders of the vacuum pump rotor and shaft. The relative axial position of the vacuum pump rotor with respect to the vacuum pump chamber may also be optimized by adjusting the axial position of the shaft with respect to the vacuum pump chamber and the centrifugal pump body, in which case the vacuum pump rotor is fixedly attached to the shaft. Finally, the relative axial position of the vacuum pump rotor and the vacuum pump chamber is optimized by adjusting the vacuum pump chamber with respect to the rotor and the centrifugal pump body, for example, by adjustment screws as is further described in detail below.
In addition, ports for the admission of make-up air for the control of the vacuum pump may be provided at the rear wall of the vacuum pump. By rear wall of the vacuum pump is meant that wall which is located opposite the air inlet port and distal the centrifugal pump housing.
Axial clearances between the vacuum pump rotor and the vacuum pump chamber walls may also be adjusted by providing a rotor with rotor blades which are slightly tapered in radial direction or wherein the side walls of the vacuum chamber are slightly tapered in radial direction relative to the shaft to account for the slight bending or flexing of the shaft during operation of the vacuum pump.
The vacuum pump may also be designed so that the air inlet port and the outlet port are on the same site of the pump within the intermediate plate and the rotor central portion is tapered conically toward the gas outlet of the pump so as to prevent the formation of a gas pocket around the rotor central portion.
The pump of the present invention is also provided with means for introducing a sealing liquid to the clearances between the vacuum pump rotor and adjacent side walls for sealing the same and thus increasing the pumping action of the device. The sealing liquid may be introduced separately to one or both sides of the vacuum pump chamber so that it can flow into and seal the space or clearance between the pump rotor and adjacent side walls of the vacuum pump. The sealing liquid may also be fed to the spaces through a single conduit leading through the central portion of the vacuum pump rotor. A control valve for regulating the vacuum of the vacuum pump may also be directly attached at the end of the make-up air channel.